Rechargeable batteries are of ever increasing importance as power supplies for a variety of items. One important class of rechargeable batteries comprises rechargeable lithium batteries, and as used herein, the term is understood to include all types of rechargeable lithium and lithium ion batteries.
The cathode is an important component of a lithium battery. During discharging and charging of the battery, lithium ions are inserted into and removed from the bulk of the cathode material respectively. Battery performance in terms of capacity, charging rate, discharge rate and life time will depend, among other things, upon the material properties of the cathode material. Some important parameters for a cathode material are its electrical conductivity and lithium transport properties. Stability is also important, as is cost and ease of fabrication.
Prior art cathode materials for lithium batteries are primarily based on metallic oxides, and in particular on oxides of nickel, manganese and cobalt. Representative cathode materials widely employed in the prior art include LiMn.sub.2 O.sub.4, LiCoO.sub.2 and LiNiO.sub.2. In some instances, metal nitrides have been utilized in electrodes. U.S. Pat. Nos. 5,589,298 and 5,702,843 disclose the use of specific lithium metal nitrides as cathode materials for lithium batteries, and U.S. Pat. No. 5,754,394 discloses capacitor electrodes composed of metal nitrides.
As will be detailed hereinbelow, the present invention is directed to a unique class of ceramic materials. The base material thereof is generally a transition metal nitride, although it is to be understood that some proportion of oxygen may be present in the material, and in particular embodiments of the present invention, oxide or oxynitride materials are important dopants for the ceramics of the present invention. The materials of the present invention have very good electrical conductivity. Lithium is highly diffusive in the materials of the present invention, and they readily and reversibly allow for the insertion of lithium therein. The materials have a very stable lattice structure and this structure is preserved through repeated charge and discharge cycles. As will be further explained hereinbelow, the materials of the present invention include a dopant species which greatly enhances the beneficial properties thereof. These and other advantages of the invention will be apparent from the discussion, description and examples which follow.